This invention relates to forward looking infrared (FLIR) imagers and more particularly to a FLIR imager having an hybrid optical/electronic processor for automatic dynamic range control and enhancement of forward looking infrared (FLIR) imagery.
A major problem exists in the display of infrared (IR) imagery on standard black and white cathode ray tube (CRT) monitors in that a typical infrared (IR) scene can have a very large dynamic range--in excess of 1000 to 1 --while a cathode ray tube (CRT) has a dynamic range of about 200 to 1. If the fine details of the IR image is to be observable above the CRT noise, the signal gain at this point must be increased. But increasing the signal gain at this point can drive hot regions of the scene into saturation and blank-out cold regions. Detail in the hot and cold regions can be observed by adjusting the display level control, but the adjustments are mutually exclusive (raising the display level to observe cold detail drives more of the hot regions into saturation while lowering the level to observe hot detail blanks out more of the colder portions of the image). If the full range of scene temperatures from hottest to coldest is to be displayed on the linear portion of the CRT transfer characteristic (without saturation or blanking), then the signal again must be reduced to the point that scene detail will be lost. Thus, extracting information from an IR scene on a CRT monitor is a continuous hands-on process of adjusting display gain and level controls.
In the past techniques to automate the control of gain and display level control have included digital histogram modification techniques to redistribute the signal values within the dynamic range capability of the display. Discussions of this technique are contained in articles entitled "Image Processing By Digital Computer", H. C. Andrews, A. G. Tescher and R. P. Kruger, IEEE Spectrum 9. p. 20, July 1972; and "Almost Uniform Distributions for Computer Image Enhancement", E. L. Hall, IEEE Trans. on Computers C23, p. 207 (1974) The redistribution can be either a compression bringing large temperature variations within the display capabilities or an expansion adjusting very bland scenes to fill the display dynamic range. Histogram modifications act like nonlinear gain characteristics based on global scene temperature distributions. As a result, detail in small localized hot and cold regions (such as targets) can be lost while the more prevalent background detail is enhanced. The digital processors could be redesigned to perform histogram modification based on local statistics to preserve detail throughout the scene. The problems with this approach are the significantly more complicated digital hardware and/or the sacrifice of real-time operation.
A strictly analog enhancement technique called automatic low frequency gain leveling (ALFGL) was developed at the Night Vision Laboratory, Fort Belvoir, Va. The algorithm was developed by Sen-Te Chow. This system incorporates dynamic range limiting circuitry between the IR detectors and the LEDs used for visible image formation. In this technique the signal is clipped to limit the range of signal swings and then image detail is added back to the clipped regions. The ALFGL system operates in real-time on small targets as well as large background regions. The one-dimensions (1-D) nature of the electronic processing circuitry requires the electronic filters for the extraction of image detail to be causal linear phase filters. These filters have non-symmetrical impulse responses that produce noticeable non-symmetrical blurring or smearing of the processed image in the scan direction. Further, as the processing occurs before the visible image reconstruction, in two-way scan FLIR systems, a zig-zagged effect along any structural line that crosses the scan direction.